Hillary Clinton, US secretary of state, on Wednesday said the US would offer additional briefings and information to China to convince it that North Korea torpedoed a South Korean warship, killing 46 sailors in March.

Beijing’s response to the sinking has been guarded and this makes it less likely South Korea will be able to marshal support for tougher sanctions at the United Nations Security Council.

Mrs Clinton said a 400-page technical report on the sinking by an international team, including experts from the US, led to the “inescapable” conclusion North Korea was to blame and that action had to be taken. Should the Chinese need more information, she said the US would offer it.

“We hope China will take us up on our offer,” she told reporters in Seoul, where she was on a one-day visit. “I believe that the Chinese understand the seriousness of this issue and are willing to listen to the concerns expressed by both South Korea and the United States.”

She also said the US could take additional measures against Pyongyang but she did not specify what they might be.

China had not reacted to Mrs Clinton’s comment but earlier in the day, the foreign ministry in Beijing repeated its call for calm and restraint from both sides on the Korean peninsula. It also said China had no first-hand information on the sinking of the South Korean ship, according to Reuters.

Wen Jiabao, the Chinese premier, is due to arrive in Seoul for a visit in Friday.

Mrs Clinton’s remarks came shortly after several hundred South Korean workers were allowed to travel to factories in the North. This suggested the most important trade link between the two Koreas, an investment enclave in the North Korean city of Kaesong, could survive rising tensions despite North Korea’s threat to sever ties with Seoul.

However, hopes of business as normal have been put in jeopardy by a North Korean threat to close access to the area if South Korea delivers on its threat to blast propaganda across the border with megaphones.

Managers from South Korea run about 100 small enterprises in the North Korean city of Kaesong, making products such as clothes and watches.

In response to North Korea’s alleged torpedoing of the warship on March 26, Seoul announced this week it was severing all trade ties apart from those with Kaesong. The trade zone represented about 56 per cent of trade between the two Koreas last year.

North Korea, which denies the attack, said on Tuesday it was freezing all ties with the South. Pyongyang said it would expel all South Korean staff from a key liaison office at the Kaesong business park. The investment enclave had represented one of the few points of diplomatic contact between North and South.

South Korean companies took $418m of goods into Kaesong last year and exported some $523m, according to data from the unification ministry.

Although North Korea is permitting South Korean workers to enter the park, Seoul is gradually reducing operations there. Kaesong often has as many as 1,000 South Korean workers but numbers have dropped this week to about 820.

“We are asking companies operating in the Kaesong Industrial Complex to reduce staff there for safety reasons,” said a spokeswoman for the unification ministry.

Political analysts have warned that any attempt to close Kaesong by the South could push the North Korean army to close the border, effectively taking any South Korean workers hostage. Still, many South Korean conservatives have favoured keeping the zone open despite the torpedo attack, seeing its potential for undermining communist ideology.